The present invention relates to a magnetic random access memory (MRAM) in which a memory cell is constituted by a TMR element for storing data values “1” and “0” in accordance with the tunneling magneto resistive effect.
Recently, many memories for respectively storing data in accordance with a new principle are proposed. One of the memories is a memory using the tunneling magneto resistive (hereafter referred to as TMR) effect, which is proposed by Roy Scheuerlein et al (refer to ISSCC2000 Technical Digest p. 128 “A 10-ns Read and Write Non-Volatile Memory Array Using a Magnetic Tunnel Junction and FET Switch in each Cell”).
A magnetic random access memory stores data values “1” and “0” by a TMR element. As shown in FIG. 1, the TMR element has a structure in which an insulating layer (tunneling barrier) is held by two magnetic layers (ferromagnetic layers). The data stored in the TMR element is determined by the fact whether spin directions of two magnetic layers are parallel or anti-parallel.
In this case, as shown in FIG. 2, “parallel” denotes that spin directions of two magnetic layers are parallel and “anti-parallel” denotes that spin directions of two magnetic layers are opposite to each other (direction of the arrow shows a spin direction).
In general, an anti-ferromagnetic layer is formed on one of two magnetic layers. An anti-ferromagnetic layer is a member for easily rewriting data by fixing the spin direction of one magnetic layer and changing only spin directions of the other magnetic layer.
As shown in FIG. 2, when spin directions of two magnetic layers become parallel, the tunneling resistance of an insulating layer (tunneling barrier) held by these two magnetic layers is minimized. This state is referred to as “1”-state. Moreover, when spin directions of the two magnetic layers become anti-parallel, the tunneling resistance of the insulating layer (tunneling barrier) held by these two magnetic layers is maximized. This state is referred to as “0”-state.
Then, the principle of write-operation to a TMR element is briefly described below by referring to FIG. 3.
A TMR element is set to the intersection between a word line WWLI and a data selection line (bit line) BLj which intersect with each other. Then, write is achieved by supplying current to using the write word line WWLi and data selection line BLj, using a magnetic field generated by the current circulating through the both lines, and thereby making spin directions of TMR elements parallel or anti-parallel.
For example, only a current flowing in one direction is supplied to the data selection line BLj and a current flowing in one direction or other direction is supplied the write-word line WWLi in accordance with write data under write. When supplying the current flowing in one direction to the write word line WWLi, spin directions of a TMR element become parallel (“1”-state). When supplying a current flowing in other direction to the write word line WWLi, spin directions of the TMR element become anti-parallel (“0”-state).
A mechanism for spin directions of a TMR element to change is described below.
As shown by the TMR curve in FIG. 4, when applying a magnetic field Hx in the major-side (Easy Axis) direction of a TMR element, resistance values of the TMR element change by approx. 17%. The change rate, that is, the ratio between resistance values before and after change is referred to as an MR ratio.
MR ratios are changed due to the property of a magnetic layer. A TMR element having an MR ratio of approx. 50% is obtained at present.
A synthetic magnetic field of a magnetic field Hx in Easy-Axis direction and a magnetic field Hy in Hard-Axis direction is applied to a TMR element. As shown by full lines and broken lines in FIG. 4, intensities of the magnetic field Hx in Easy-Axis direction required to change resistance values of the TMR element are also changed due to the intensity of the magnetic field Hy in Hard-Axis direction. By using this phenomenon, it is possible to write data in only a TMR element present at the intersection between a selected write word line and a selected data selection line among memory cells arrange like an array.
The above state is further described below by using the ateroid curve in FIG. 5.
The full line in FIG. 5 shows the asteroid curve of a TMR element MR1. That is, when the intensity of the synthetic magnetic field of the magnetic field Hx in Easy-Axis direction and the magnetic field Hy in Hard-Axis direction is present at the outside (e.g. position of a black circle) of the asteroid curve (full line), it is possible to invert the spin direction of a magnetic layer.
However, when the intensity of the synthetic magnetic field of the magnetic field Hx in Easy-Axis direction and the magnetic field Hy in Hard-Axis direction is present at the inside (e.g. position of a white circle) of the asteroid curve (full line), it is impossible to invert the spin direction of a magnetic layer.
Therefore, it is possible to control the write of data to a TMR element by changing intensities of the magnetic field Hx in Easy-axis direction and the magnetic field Hy in Hard-axis direction and changing positions of the intensity of the synthetic magnetic field in the Hx-Hy plane.
It is possible to easily perform read by supplying a current to a selected TMR element and detecting the resistance value of the TMR element.
For example, a current path is formed by connecting switching elements to a TMR element in series and turning on only a switching element connected to a read word line. As a result, because a current is supplied to only a selected TMR element, it is possible to read data from the TMR element.
In recent years, it has been an indispensable art to increase the capacity of a memory.
In the case of a conventional memory, to increase its capacity, the memory cell area of an element is decreased by fining the element, memory cells are three-dimensionally arranged, or data of three values or more (or data of a plurality of bits) is stored in memory cells.
However, fining of an element is limited. Moreover, in the case of a magnetic random access memory, a memory cell conventionally includes only one TMR element. Furthermore, the TRM element is constituted by one insulating layer (tunneling barrier) and two magnetic layers (ferromagnetic layers) hosing the insulating layer.
That is, because a TMR element can have only two states in which spin directions of two magnetic layers are parallel and anti-parallel, a memory cell can only store one-bit data.